What was the Berlin Airlift, and why did it happen?

The Berlin Airlift refers to the massive program of
airlifting supplies of essential goods to West Berliners undertaken from June 1948 to
September 1949. This program was undertaken when the Soviet Union blocked all rail,
water, and road routes through the Soviet zone to the Western sectors of Berlin. After
the World War II, Eastern Part of Berlin was controlled by USSR, while the rest was
under control of USA, UK and France. Entire Berlin was located within the East German
territory controlled by USSR. It was expected that USSR will permit free access to West
Berlin to others.  However, in June 1948, they blocked all rail, water, and road routes
through East Germany to the Western sectors of Berlin, hoping to drive Western troops
out of Berlin. The Berlin Airlift was organized to break this
blockade.

At the height of Berlin Airlift project planes
landed in West Berlin at the rate of one every one to two minutes. As a result USSR
ended the blockade in May 1949, and the airlift stopped in September
1949.